Lamp dimming system



y 13, 1941- G. A. BRET' I'E LL, JR 2,242,105 LAMP IJIMMING SYSTEM Filed Aug. 24, 1939. 2 Sheets-Sheet 1 poms-2 4/: mm;

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LAMP DIMMING SYSTEM May 13', 1941.

2 Sheets-Sheet 2 Filed Aug; 24, 1939 I 36) I 35 W POWER sauecs 1 awe-24ml:

GEORGE A. JR'TTELLJR.

. INVENTOR Patented" May 13, 1941 LAMP DIMMING SYSTEM George Alvin Brettell, (in, Newark, N. J., assignor to' American Transformer Company, Newark,

Application August 24 1939, Serial No. 291,685

I 6 Claims. (Cl. 175-312) This invention relates to the transmission of lighting current to electric lamps, and more particularly to the control of the lighting current for the purpose of dimming the lamps. The invention is applicable to the dimming of lamps used for a variety of purposes, and a particular example is the dimming of theater lamps.

To dim theater lamps or the like by means of series rheostats, requires bulky and expensive equipment, involves largepower loss, and provides only make-shift control. Variations in the size of the lamp load correspondingly affect the degree of dimming produced by a given rheostat setting. To dim with reactors or variable transformers involves similar difilculties, in more or less degree, and makes it particularly burden- 1 some to provide progressive dimming from full relay arrangements have not achieved a desirable degree of mechanical and electrical simplicity; nor havethey reduced the cost of equipment and the amount of power loss to the extent,

that is desirable.

Among the general objects of the present invention are to simplify and reduce the cost of dimming equipment; to provide for low power loss; and to provide for flexible and positive dimming control.

Another object is to achieve the foregoing results with equipment which can be substituted in remote 'control systems that are now in common use.

A further object is to provide dimming appa-' ratus which delivers controlled voltage to the lamp-load, which voltage is substantially independent of variations in the size of the lamp load within the rated capaeityof the dimming apparatus. I v

A still further object is to provide dimming apparatus which completely eliminates the flow of current in the lamp circuit when pilot control power is zero or at a minimum.

Various other objects and advantages will be apparent to those skilled in the art from the following disclosure. I

Fig. 1 of the drawings shows voltage curves tubes with ignition control. Of the tubes which illustrating electrical principles utilized in the present invention.

Figs. 2 and 3 are voltage curvesillustrating electrical changes involved in the dimming operation.

Fig. 4 is a schematic diagram showing dimming apparatus embodying the present invention.

Fig. 5 is a wiring diagram illustrating one method of paralleling thedischarge tubes.

Reference will first be had to Figs. 1, 2 and 3.

Wave III is a sine wave corresponding to the voltage delivered by the conventional alternating current power supply. The lamp load is connected to such a power supply, but in series with the load there is connected a pair of electric discharge tubes each having an anode, a cathode,

and control means. Each tube is normally nonconducting, i. e., it blocks the flow of current until the control means becomes eflective to fflre the tube, 1. e., render it conducting. Once a tube is fired it remains conducting during the remainder of the power half cycle, 1. e. until the current wave passes through zero. Thus, if a tube is fired at time t1 (Fig. 1) current will flow to the lamp load during theinterval t1 to is and then be blocked out again. The tubes are asymmetric conductors, and hence can conduct current in one direction only. However, by using a pair of tubes, one tube can be connected to fire and then transmit lighting current during an end fragment of each positive power pulse, and

the other tube can be connected to fire and then transmit lighting current during an end fragment of each negative power pulse. The net result of suchaction is to energize the lamp load with a non-sinusoidal alternating current correspending to the non-sinusoidal altemating-voltage represented by curve ll. Voltage curve vll is spaced from voltage curve III by an amount equal to the voltage drop across the tube when the tube is conducting. By shifting the time of firing with respect to wave [0, different sized fragments of the half cycles in wave III are applied to the lamp load, thereby varying the effective energization of the lamps.

The electric discharge tubes that produce wave i! may be of. various types which permit of adequate control of the discharge stream to thereby control the time of firing. Among such types of tubes are hot cathode gaseous or vapor tubes with grid or magnetic control, and mercury pool are commercially available at present; I prefer Xenon-filled, hot cathode, grid controlled tubes.

When I refer to-using a pair of tubes. or two tubes, I also include a case where a single envelope may be used to inclose the structure funcapplied to the grid; and when the axis of wave l2 rises to the axis of wave l0, as shown in Fig. 2, the tube fires and conducts during the remainder of the half-cycle of wave l0. Thus the shaded portion of wave III of Fig. 2 represents the fragment of the positive powerpulse which is rendered effective to send current to the lamp load. Fig. 3 illustrates the effect of depressing control wave I2, e. g., by also impressing upon the grid a negative D. C. bias 1:. The effective fragment.

of wave I is diminished as shaded portion in Fig. 3.

The system is so set up that full lamp brilliancy is obtained'without utilizing all of power wave l0. By shifting the control wave i2, progressively smaller fragments of wave ID are applied to the lamp load, thereby progressively dimming the lamps. It will be apparent that continued depression of wave i2 will result in that wave not ascending to the axis until the point where wave ID has descended to theaxis. Then the shaded area (e. g. in Fig. 3) will vanish and no'current will be delivered to the lamp load. Instead of elevating and depressing control wave l2 by means of D. C.,bias, the phase of wave l2 (with respect to wave can be shifted to accomplish the same result, or a peaked wave can be used where the time of the peak is changed.

Reference will now be had to Fig. 4. This figure contains various duplicated elements symmetrically arranged. In general only one set of elements will be described, but the same reference characters with a prime mark added thereto will be applied'to the duplicate elements.

A lamp load I! is connected to a conventional source of alternating current I]. In series with the lamp load is a pair of gaseous rectifier tubes I1 and I1, one to transmit'fragments of the positive power pulses and the other-to transmit fragments of the negative power pulses. Connected in bridge of source I8 is a transformer i8 having secondary windings l9 and 20. Winding I9 is connected to energize the cathode of tube l1; and winding energizes a closed loop 2| from which a control voltage wave (such as wave I 2 in Figs. 2 and 3)' is derived to influence the grid of tube IT. The. lag of the control voltage isproduced by condenser 22 and resistance 24. An auxiliary rectifier 25, also energized from winding 20, furnishes a fixed D. 0. negative bias for the grid of tube l1, this bias being sufllcient to block the tube in spite of the effect of the control A. C. wave. Thus, in the absence of a definite pilot voltage to set the tube ll inoperation, such tube remains blocked and no current flows to load IS. The pilot voltage is furnished from full wave'rectifier 26 and is adjustable to overp wer the bias from auxiliary rectifier 25, and thus elevate and depress the A. C. control wave to thereby regulate the eflective current transmitted to the lamp load as explained in connection with Figs. 2 and 3.

The circuit for auxiliary rectifier 25 is from the center tap of winding 20frectifier 25, loading resistance 21 which is shunted by smoothing conindicated by the denser 28, and back to the right-hand end of winding 20.

Full wave rectifier 26 is energized from a transformer 29 having a variable input connection at 30. As shown, the input of transformer 29 is connected for energization from the power source IS. The output terminals of full wave rectifier 26 are connected to the center taps of the isolating choke 3i-32.- Bridged across the output of rectifier 26 is a loading resistance 34 which is shunted by a smoothing condenser 35.

The grid circuit for tube I1 is from the grid thereof, isolating resistor 36, left half of winding 3|, resistance 34, left half of winding 32, resistance 24, resistance 21, center tap of filament winding l9, and cathode of tube l1. Similarly, the g'ridcircuitfor tube I1 is from the grid thereof, i'solatingsgzresistor 36', right half of winding 3|, resistance 34, right half of winding 32, resistance 24, resistance 21', center tap of filament winding l9, and cathode of tube II. It will be noted that these grid circuits, are not blocked by condensers, and therefore they can impress D. C. bias as well as A. 0. potential on the grids.

When tube I1 is conducting (during a power pulse in one direction from source It) lighting current flows from power source l8, conductor 31, lamp load l5, conductors 38 and 39, anode of tube I1, cathode of tube ll, filament winding is, center tap of winding l9, and conductors 40, 4|, and 42 back tosource is. When tube 11' is conducting (during a power pulse in the opposite direction from source i6) lighting current flows from power source It, conductors 42, 4|, and 39, anode of tube ll. cathode of tube ll, filament winding l9, center tap of winding l9, conductors '40 and 38, lamp load I 5, and conductor 21 back to source i6.

Assume the adjustment at 30 to be in a position which gives zero input to transformer 29. The lagging A. C. control voltage-drops across resistances 24 and 24' are impressed upon the grids of tubes I! and I1 respectively, since these resistances are in the respective grid circuits. However, these grid circuits also contain resistances 21 and 21', respectively, through which direct current flows from 'auxiliary rectifiers 25 and 25', respectively; and the D. C. voltage-drops across resistances 21 and 21' keep the tubes l1 and I1 blocked. Thus, no lighting current flows; and it will be noted that in this condition the pilot input to transformer 29 is also zero. As adjustment 30 is moved to increasingly energize :transformer 29, a progressively increasing positive bias is added to the two grid circuits by the increasing voltage-drop across resistance 34. This positive bias overpowers the blocking biases from auxiliary rectiflers 2! and 25' and elevates the axes of the respective control voltage waves impressed on the grid circuits by resistances 24 and 24'. 'Ihuathe lamp load is increasingly energized as will be clear from the explanation given inconn'ection with Figs. 1, 2 and 3.

The ohmic resistance of a ,rheostat is independent of the current passing through it, so the voltage-drop which it imposes on a series circuit is equal to the rheostat resistance multiplied by the eflective load current. Thus, the dimming eilect'of a series rheostat varies drastically with the load. With a gaseous conductor I such as tubes i! and H, the ohmic resistance decreases as "the load current increases; and the decrease issuch'that the voltage-drop imposed on the load circuit is substantially independent of the load current. Thus, the system of the present invention gives uniform dimming regardless of the size of the lamp load, provided only the wave form of the current. However, because of the resistance characteristics of tubes l1 and l I, the heating eifect of the load current on each tube is determined by the integrated average value of the current passing through the ,tube (which does not involve reversal of the wave). The integrated average value of the composite load current (which involves wave reversal) is zero. Thusgbyusing a wave form which has a root-mean-square value that is high in com-' parison to its average absolute value, the effecpresent invention.

The ratio of the integrated root-mean-square positive with respect to G2, a signal entering at s will firetube 41 in preference to tube 41. Thus, if a source of 3.0-cycle voltage 43 be inserted at M and the plate and grid supplies be 60-cycle voltage, the tubes will alternate in. carrying the load current. On the first cycle of the ,60-cycle power supply tube 41 will fire while on thesecond cycle tube 41' will fire. The average load current will thus .be split between the two tubes, enabling the combination to control twice the lamp capacity that one tube could. This partic ular system has the advantage of eliminating the bulky and expensive balance coil normally required.

In compliance with the patentstatutes I'have disclosed the best forms in which I have contrative only; and it will be apparent to those value to the integrated average value of a half cycle is' commonly known as the form factor of the wave. For a pure sine wave the form factor However,

given effective lighting current delivered to the lamp load.- As a numerical example, a rootmean-square current of 25 amperes delivered to the lamp load at the sine wave form factor of 1.1 would mean that the two tubes together had to handle an average current of 22.8 amperes,

whereas the same root-mean-square current delivered at a form factor of 2.0 would burden the two tubes together with an. average current of only 12.5 ampercs' Thus by so operating that wave N (Fig. 1) has a high form factor, the load carrying capabilities of tubes I1 and I1 are enhanced. In general, the nearer the point of fire occurs to the end of the half cycle of the wave, the higher the form factor of the load current.. Thus, from this standpoint, it is desirable to utilize only a fragment of each power cycle even when the lamps are turned fully on. Under all dimmed conditions the load current will be less and the duty imposed on tubes l1 and I1 will be correspondingly less.

The load carrying capabilities of the apparatus may be augmented by adding a duplicate tube connected in parallel withtube l'l, together with a second duplicate tube connected in parallel withtube ll. .Tube 11 and its duplicate will then fire. simultaneously, a balance coil being used to divide the load equally between the two tubes. Similarly, tube l1 and its duplicate will fire simultarieously,-'each tube carrying half the load. Another way of utilizing the parallel arrangements of tubes is to arrange the grid circuits to fire tube H on one half cycle, and fire skilled in the art that various substitutions, changes, and mutations may be made within the scope of the invention.

What is claimed is:

1. Apparatus for dimming electric lamps energized from a course of alternating current, said apparatus comprising a plurality of rectifying electric discharge devices connectedto transmit the lighting current to the lamp load, said de vices operatingto deliver current having a form factor greater than 1.1 at maximum root-meansquare load current; and circuit connections protecting the load and source from any net direct current resulting from the rectifying action of said discharge devices.

2. Apparatus for dimming electric lamps energized from a source of alternating current,'said apparatus comprising electric discharge devices connected to control the lighting current deliv- 'ered to the lamp load, said devices having means to initiate their conductivity, means to bias said.

conductivity-initiating means to normally block said discharge devices and means to overpower said blocking biases and thereby fire said devices and cause lighting current of form factor sensibly greater than 1.1 to be delivered to the lamp load.

3. Apparatus for dimming electric lamps energized from a source of alternating current, said apparatuscomprising gaseous electric discharge devices connected to control the lighting current 'delivered to the lamp load, said devices having electrodes the potential of which initiates the conductivity ,of the devices, means to bias said electrodes to normally block said discharge dey vices and control means operative to overpower said blocking biases and thereby fire said devices and cause lighting current of form factor sensiapparatus comprising a pair of rectifying. gas-' eous electric dischargedevices having electrodes the potential of whichinitiates theconductivity of the devices, one of said devices being connectedto deliver current to the lamp load during positive half cycles of the power source and it the other being connected to deliver current to the lamp load during negatiwe half cycles .of the power source; auxiliary rectifiers conneicted, to

the tubes may beparalleled to fire alternately. 7

Here the plates PI. and P2 of the two tubes 41, 1', are directly connected together, as are also the cathodes C I and C2. Now, if Gl'is-slightly bias saidelectrodes to normally block said discharge devices and a control rectifier and sources of control alternating current connected 'to'overpower the bias imposed by each 'auxi-liaryrecti tier and also render each discharge device conducting to transmit current of f orm'factor sensibly greater than 1.1 to the lamp load.

5. Apparatus for dimming electric lamps energlzed from a source of alternating current. said apparatus comprising a pair of rectifying gaseous electric discharge devices having electrodes the potential of which initiates the conductivity of the devices, one of said devices being connected to deliver current to the" lamp load during positive half cycles of the power source and the other being connected to deliver current to the lamp load during'negatlve half cycles of the power source, auxiliary rectiflers connected to bias said electrodes to normally block said discharge devices, a control rectifier and sources 7 of control alternating currents connected to overpower the bias imposed by each auxiliary recti- 2,242,1o5 v I i 6.Light control means as herein disclosed,

comprising electric discharge devices having parallel connected anodes and control electrodes, means for impressing-an alternating current voltage on said parallel connected anodes and means for impressing an auxiliary signal between the'two connected control electrodes of a fre- 'quency one half that of the anode voltage Irequency to effect alternate firing of said discharge devices.

GEORGE ALVIN BRE'II'EIL, Ja 

